Why Does My Left Outer Join Only Return Two Rows Instead of Thirty?

Troubleshooting a Left Outer Join: Why Only Two Rows Were Returned

A user attempted to count daily page views using a left outer join, but the query unexpectedly returned only two rows instead of the expected thirty. Let's examine the problematic query and its solution.

The original query:

SELECT day.days, COUNT(*) AS opens
FROM day
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date))
WHERE tracking.open_id = 10
GROUP BY day.days;

The Problem: WHERE Clause After the JOIN

The issue lies in the WHERE tracking.open_id = 10 clause. This condition is applied after the left outer join. A left outer join is designed to include all rows from the left table (day), even if there's no match in the right table (tracking). However, the WHERE clause effectively filters out any rows where tracking.open_id is not 10, thus eliminating rows from the day table that lack a corresponding entry in tracking with open_id = 10.

The Solution: Integrating the Condition into the JOIN

To retrieve all thirty rows from the day table, the filtering condition needs to be part of the join condition itself. This is achieved using AND:

SELECT day.days, COUNT(tracking.open_id) AS opens
FROM day
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date)) AND tracking.open_id = 10
GROUP BY day.days;

Note the change to COUNT(tracking.open_id): This ensures that days without matching entries in tracking will have a count of 0, rather than NULL, which would cause issues with some database systems. This corrected query correctly performs a left outer join and returns the expected thirty rows, accurately representing daily page opens.

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